1. Field of the Invention
The present invention relates to an apparatus and method for controlling ignition timing for an internal combustion engine, and in particular, to such apparatus and method capable of suppressing variations in speed of the internal combustion engine.
2. Description of the Prior Art
Recently, a phenomenon of irregular vibrations has been noticed as a problem in which, in an automobile having an internal combustion engine mounted thereon, the irregular vibrations (so-called surging) are caused in the automobile when a resonant frequency determined by the internal combustion engine and the car body matches the engine rotational speed (rpm) of the internal combustion engine.
In order to eliminate this phenomenon, for example, as disclosed in U.S. Pat. No. 4,527,523, it has been proposed to control the ignition timing.
However, in the case of this U.S. Patent, since it is arranged to detect an actual engine speed (rpm) through a filter, and to compare the actual engine speed with an engine speed processed through the filter thereby to provide discrimination, it is impossible to detect a variation in the engine speed at an early time. And furthermore, since a time point at which the ignition timing is corrected and the amount of correction are not optimum, the effect of suppressing the vibrations has not been so satisfactory as expected.